Temperature change responsive resistance measuring and protecting systems have been known heretofore. For example, F. E. Murphy, Jr. U.S. Pat. No. 3,417,330, dated Dec. 17, 1968, discloses a system for indicating current flow proportional to change in resistance of an electrical heating element, a resistor or the like. This current flow may be used to operate an indicator such as a milliammeter or a control device such as an impedance element, an electric relay coil, lamp or alarm, or an electronic switch. Such resistance change is proportional to the temperature of the heating element and thus the system may be used to regulate the temperature of the heating element. These prior systems have had certain disadvantages such as requiring high power comsumption for the sensing and control device, being rather insensitive to small changes in the condition to be sensed, the control device being subject to unwanted temperature variation or subject to vibration, or the like.
While these prior systems have been useful for their intended purpose, this invention relates to improvements thereover.